1. Field of the Invention
The present invention relates to a method of forming a fine bore suitable for use in the production of a head for an ink jet plotter and the like.
2. Description of the Prior Art
The nozzle bore formed in the head of ink jet plotter usually has a diameter of about 30.mu. or so. Various methods have been taken for forming such fine bores, such as photo-etching, electro-forming, mechanical processing, laser beam, electronic beam and so forth. Referring first to the photo-etching method, the formed bore inconveniently has a conical shape or the bore is spread at both its ends. In addition, it is not possible to obtain a sufficiently high linearity at the side surface of the bore. Due to the problem concerning this side edge, the kind of material and the plate thickness are impractically limited for achieving sufficiently high precision work.
The electro-forming also cannot provide a sufficiently high cylindricalness because of melting of material at both ends of the bore, and cannot permit easy formation of a fine bore with high precision.
In the case of mechanical processing, the fine bore is formed by means of a micro-drill or like tool. In this case, however, the yield rate is impractically low and the bore is liable to be tapered due to eccentricity or offset of the tool. In addition, the drill, which is expensive, is worn out rapidly. Further, the mechanical processing with a drill necessitates an additional step of removal of burr.
The method making use of a laser beam or electron beam has a drawback in that the initial cost is impractically high and that the secondary work such as removal of thermally-affected layer and shaping of the bore is essential, and cannot provide for easy formation of the bore.
For these reasons, it has been proposed and attempted to form a fine bore by surrounding a core wire with a metal, embedding the metal and the core wire as a unit in a plastic and finally removing the core wire by melting, dissolving or by an electrolytic process, thereby forming a bore of the same diameter as the core wire. In such a case, since the matrix material is a plastic which has little durability, it is necessary to remove only the core wire, leaving the metallic layer formed on the core wire. As a result, the whole process is inconveniently complicated and the step of removal of the core wire has to be made under various restrictions. The removal of the core wire by a chemical process is usually accompanied with production of noxious substances, requiring countermeasures for the treatment of effluent or waste water.